This invention relates generally to an improvement in the recharging of membrane type polaragraphic oxygen sensors. Specifically, this invention is an improvement over the cap assembly described and claimed in U.S. Pat. No. 3,666,650, issued May 30, 1972, which is incorporated by reference in its entirety herein. Other patents describing earlier devices related to the invention herein include U.S. Pat. Nos. 2,913,386; 3,518,179; 3,445,369; and 3,211,638.
As discussed in U.S. Pat. No. 3,666,650, the "life" of the membrane cap assembly of oxygen monitors of the type disclosed therein depends upon the life of the quantity of electrolyte retained in the cap assembly. That is, when the electrolyte is depolarized, it is necessary to remove the cap assembly, to clean the electrodes, to incorporate a new cap assembly with a new membrane and a fresh quantity of electrolyte. This is a tedious procedure, at best. Therefore, it is desirable to reduce to as few times as possible the necessity of removing the cap assembly and going through the replacement and cleaning procedure. It follows, therefore, that the nature and quantity and handling of the electrolyte, upon installation, is most important so that the electrolyte is depolarized less frequently.
In U.S. Pat. No. 3,666,650 referred to above, a pre-assembled cap assembly is provided wherein the membrane is a disc with the outer circumferential edges thereof secured between the cap body and a retainer ring. Thus, when the cap body is assembled onto the electrode body by cooperating threads between the two bodies, the extended electrodes have the effect of stretching the mounted membrane to accommodate the inserted electrodes. This stretching procedure has the effect of reducing the quantity of electrolyte between the membrane and the inserting electrode surfaces. Thus, the electrolyte is partially squeezed, as discussed in that patent out of the area defined by the membrane on one hand and the electrodes on the other hand. This, in turn, reduces the life of the cap assembly. While such an assembly is a vast improvement over the assemblies utilizing many parts developed previous to the assembly of U.S. Pat. No. 3,666,650, the life of the assembly between changing and cleaning operations is not as long as desired.
With this invention by contrast, an improved membrane cap assembly is provided in which the membrane disc is secured between the cap body and a retainer in such a manner that the outer circumferential portion of the disc is slidable in its secured state to accommodate and receive the electrodes upon mounting the cap assembly to the sensor body so that the membrane is not stretched. There is less force applied to the electrolyte positioned between the membrane and the inserted electrodes, and the quantity of electrolyte retained is substantially greater. This, in turn, has the effect of increasing the life of the assembly and reducing the number of times that the assembly must be changed and cleaned.
This is achieved by utilizing, as discussed above, a retainer which is press-fit into a counterbore in the end of the cap assembly opposite the bore for receiving the electrode body. The retainer is configured to have an annular portion which is spaced from the cooperating annular surface of the cap body so as to provide an annular space for receiving and retaining the outer circumferential portion of the membrane disc. This, has the effect, of allowing the outer circumferential portion of the membrane disc to slide in its retained area to the extent necessary for accommodating the electrode body when it is inserted into the cap assembly body. Thus, there is no substantial pressure applied to the membrane and the electrolyte retained between the electrode body and the membrane. This allows for a larger portion of the electrolyte to be retained between the electrode tip engaging the membrane and the membrane itself.
Other objects and advantages of this invention will be apparent from the following description, the accompanying drawings and the appended claims.